20 years is your figure, but the common one is 50 years - also quoted by notable people in the secretive aviation community like Kelly Johnson and others who worked at either the Phantom or Skunkworks
Well they should be in a better position to know than I, but I suspects the issue hinges on how we define "50 years ahead".
Perhaps they are talking about 50 years ahead of adoption by the commercial industry or some such?
I would also have to say it's not necessarily linear, and that modern companies are more willing to adopt advanced technology faster than they were in the past.
Also, I wonder if they are taking into account the non linearity of research and development? The more we learn, the faster we develop new technologies. 50 years later at a linear progression is a very different thing than 50 years later at an exponential progression.
I happen to know the Navy is very interested in Fusion propulsion systems for their ships because they had/are funding the Polywell fusion reactor research. If the technology is viable, we will likely have fusion powered ships, planes, trucks, whatever in much less than 50 years.
50 years advanced from now? It's unfathomable.
Warp drive maybe.